Measuring the Distribution and Excitation of Cometary CH3OH Using ALMA

TL;DR

This study used ALMA to spatially and spectrally resolve CH$_3$OH emission in comet C/2012 K1, revealing temperature profiles and distribution consistent with spherically symmetric outflow, and analyzing thermal variations in the coma.

Contribution

First detailed spatially-resolved measurements of CH$_3$OH in a comet's coma using ALMA, providing insights into temperature profiles and excitation processes.

Findings

CH$_3$OH distribution is centrally peaked and spherically symmetric.

Rotational temperature decreases with distance from the nucleus.

Temperature variations are mainly due to adiabatic cooling and solar heating.

Abstract

The Atacama Large Millimeter/submillimeter Array (ALMA) was used to obtain measurements of spatially and spectrally resolved CH$_3$OH emission from comet C/2012 K1 (PanSTARRS) on 28-29 June 2014. Detection of 12-14 emission lines of CH$_3$OH on each day permitted the derivation of spatially-resolved rotational temperature profiles (averaged along the line of sight), for the innermost 5000 km of the coma. On each day, the CH$_3$OH distribution was centrally peaked and approximately consistent with spherically symmetric, uniform outflow. The azimuthally-averaged CH$_3$OH rotational temperature ($T_{rot}$) as a function of sky-projected nucleocentric distance ($\rho$), fell by about 40 K between $\rho=0$ and 2500 km on 28 June, whereas on 29 June, $T_{rot}$ fell by about 50 K between $\rho=$0 km and 1500 km. A remarkable ($\sim50$ K) rise in $T_{rot}$ at $\rho=$1500-2500 km on 29 June was not present on 28 June. The observed variations in CH$_3$OH rotational temperature are interpreted primarily as a result of variations in the coma kinetic temperature due to adiabatic cooling, and heating through Solar irradiation, but collisional and radiative non-LTE excitation processes also play a role.